Field of the Invention
The present invention relates to an image processing method for suppressing the occurrence of, in particular, an image defect in an image forming apparatus of an electrophotographic system.
Description of the Related Art
In general, in an image forming apparatus of an electrophotographic system, after the surface of an image carrier (e.g., photoconductor drum) is electrified evenly by an electrifying device, a toner image is formed on the surface of the photoconductor drum. Then, the toner image that is formed on the surface of the photoconductor drum is transferred onto a printing medium, such as paper, or an intermediate image transfer body by a transfer device.
At the time of forming an image by such an electrophotographic system, there is a case where the surface potential of the photoconductor drum after transfer processing becomes uneven. In the case where the surface of the photoconductor drum whose surface potential is uneven is electrified again by the electrifying device, there is a possibility that unevenness will remain in the surface potential because it is not possible to evenly electrify the surface of the photoconductor drum. The unevenness in the surface potential such as this in the photoconductor drum may cause an image defect called a ghost to occur in an image that is formed on the photoconductor drum afterward.
As a conventional technique to eliminate the unevenness such as this in the surface potential of the photoconductor drum, there is a technique to provide an electricity eliminating device called a pre-exposure device between a transfer device and an electrifying device along the rotation direction of the photoconductor drum. According to this technique, it is possible to prevent the occurrence of an image defect by eliminating electricity from the surface of the photoconductor drum with exposure light from the pre-exposure device before electrifying the surface of the photoconductor drum by an electrifier, and thereby, shifting the surface potential to an even potential.
On the other hand, in recent years, there is a demand for an image forming apparatus that implements a low cost and downsizing. Installing the above-described pre-exposure device in the image forming apparatus increases the manufacturing cost and the apparatus size, and therefore, is incompatible with the above-described demand.
In view of this point, a technique has been proposed (see Japanese Patent Laid-Open No. 2009-109544), in which an attempt to suppress the occurrence of a ghost is made by correcting the density of image data at a downstream position, in which a ghost is expected to occur, based on a combination (correspondence relationship) of image data that is formed on the upstream side, image data that is formed on the downstream side, and an amount of correction of the image data that is formed on the downstream side in a color image forming apparatus in which no pre-exposure device is installed.
However, with the technique of Japanese Patent Laid-Open No. 2009-109544, which increases the density of the portion where a ghost occurs in the image data that is formed on the downstream side where a ghost occurs, it is difficult to perfectly match the density of the portion where a ghost occurs with the density of the portion where no ghost occurs. The reason is that in the case where the cause of the occurrence of a ghost is, for example, the fact that the image data on the upstream side is data of a complicated shape, such as character data, it is difficult to change the density of only the corresponding area portion in the image data on the downstream side because the positioning control or the like is difficult. Further, the density of the ghost occurrence portion is not perfectly even and is subject to the influence of the change with the passage of time or the environmental fluctuations even by performing correction using a test chart.